The disclosures herein relate generally to space frame joints and more particularly to a method and apparatus for limiting end-crippling tube compression in a tube element adjacent a hub connection.
Separable joints, such as those used in space frame construction are generally a cylindrical hub which includes a plurality of radially directed slots extending axially along the circumferential surface of the hub. The slots have opposed surfaces facing each other which are ribbed. The ribs extend axially along the opposed surfaces.
Tubular frame members are flattened and crimped at their opposed ends. The crimped ends include elongated flat surfaces extending outwardly, or away from each other. The crimped ends are ribbed in a pattern which can be mated into engagement with the ribs in the hub slots. In this manner, each end of the tubular frame member may be slidably inserted into a respective hub slot and several tubular frame members may be connected at one end to a hub slot to form a spider, i.e., a hub having a plurality of tubes extending radially outwardly therefrom, each tube terminating at a free end.
The free end of each tube can be similarly connected to another hub. Thus, a framework of interconnected spiders formed of tubes and hubs can be joined to form a pre-assembled or modular section of a flat roof, a doomed roof, a wall, etc., to be joined with other sections to eventually form a complete structure. The structure, once completed, is then covered with a selected cladding which is attached to the structural framework by means of an interfacing cladding support system. The cladding may be fabric, corrugated steel plates, glass, and other selected materials and may include combinations of these materials for architectural design purposes. For example, a domed roof may be clad with steel and may include a pattern of glass panels in a portion of the roof which has an aesthetic effect when viewed from the interior of the structure.
Joining the pre-assembled sections of the framework is difficult and time-consuming. Each section has a periphery including a plurality of hubs. These hubs are connected to their respective section by tube members as mentioned above. When sections are to be joined, additional members of tubing have one end connected to the peripheral hubs of one section, and have their opposite ends connected to the peripheral hubs of an adjoining section. Because the modular sections are often quite large, e.g., 50 feet square, they must be hoisted into their approximate position, suspended by a crane and then a few of the additional members of tubing must be individually and manually aligned with and inserted into the peripheral hubs to stabilize the hoisted section relative to the section of framework to which the hoisted section is being attached. Once stabilized, additional members of tubing are used to complete the interconnection of the hoisted section into the framework.
There are various known tubular structural devices which have been developed. U.S. Pat. No. 2,895,753 discloses tubular members having crimped ends matingly engaged with keyed slots for retaining the crimped tube ends. U.S. Pat. No. 2,916,109 discloses a similar tube and hub construction used to form reinforced metal wall sections. U.S. Pat. No. 2,931,467 discloses a tube and hub construction for use in fabricating scaffolds, trusses, bridges, walls, roofs and towers. U.S. Pat. No. 2,964,147 discloses a tube and hub construction directed to radial and/or diagonal struts and ties. U.S. Pat. No. 2,976,968 discloses a parallel member tubular wall construction using tubular members with flattened and crimped end sections. U.S. Pat. No. 3,079,681 discloses an improved keyed connection between the hub and the crimped tube end. U.S. Pat. No. 3,081,601 discloses tubular members connected to hubs for forming a demountable dock. U.S. Pat. No. 3,152,819 discloses a key and keyway joint for tube and hub structural assemblies. U.S. Pat. No. 3,275,351 discloses a hub having keyway connectors with integral nut receiving ends and an annular recess of conical cross-section at one end with a complimentary tapered washer. U.S. Pat. No. 3,309,121 discloses a connector for slidingly receiving formed tube ends. The connector is formed by a stack of disks having key slots formed therein.
Because the tube members are structural elements, they have tensile, compression and bending forces acting thereon. One problem caused by the forces of compression acting on the tube members is commonly referred to as end-crippling. This occurs in the tube member near the connection to the hub member, and is concentrated in an area of the tube member adjacent to where the circular tube member is formed into opposed flattened, tapered areas having a crimped end.
One attempt to overcome the end-crippling problem has been to weld a triangular shaped plate directly on each of the opposed flattened, tapered areas. The purpose of the plates is to reinforce the ends of the tube members against the end-crippling loads acting thereon. Another approach to overcome this problem is to add an external collar which attaches to the circular portion of the tube immediately adjacent the flattened, tapered areas. This also reinforces the ends of the tube members against end-crippling compression loading. However, the addition of the plates and collar involves additional parts and is a time consuming manufacturing operation which adds costs to the overall structure. In addition, these reinforcing members protrude from and obstruct the outer surface of the tube members. As a result, such obstructions can interfere with the process of applying a smooth cladding to the assembled support members.
Therefore, what is needed is an apparatus and a method which overcomes the end-crippling problems associated with compression loads on the tube members, and which provides non-protruding adequate reinforcement of the ends of the tube members which adds minimum cost and production time to the structure.